1. Field of the Invention
The present invention relates to a focus adjustment apparatus used in a camera, for example.
2. Description of the Related Art
Conventionally, a camera focus detection apparatus has been known that stores object image signals that have passed through an imaging lens in an autofocus (AF) sensor, and detects a defocus amount based on the object image signals stored in the AF sensor.
In such a focus detection apparatus, when continuously focusing on a moving object, a prediction calculation function (AI-servo) moves the imaging lens by performing focus detection for each frame in continuous shooting to predict a future movement position of the object.
As illustrated in FIG. 20, the AI-servo calculates an amount deviating from the object by determining a defocus amount by performing a correlation calculation based on signals stored in the AF sensor. Next, the AI-servo performs a prediction calculation for predicting a future image plane position of the object, based on the determined focus detection result and past focus detection results, and drives the lens by an amount that takes into account the focus detection result and the prediction calculation result to continue the focus on the moving object.
Japanese Patent Application Laid-Open No. 2001-21794 discusses in detail control for driving an imaging lens in conjunction with movement of a moving object.
In the present invention, the operation from storage of the object image signals in the AF sensor, defocus amount detection by correlation calculation, up to prediction calculation is referred to as a focus detection operation. Generally, the focus detection operation is completed while the mirror is down, and imaging lens drive and aperture reduction are performed during the period from mirror up until exposure. This is because since it takes a predetermined amount of time to drive the lens and reduce the aperture, if the focus detection operation is not completed while the mirror is down, a finder vanishing time is extended, which makes it difficult for the user to track the object. In the present invention, the time that the mirror is down is referred to as AF possible time.
For the camera to maintain a continuous shooting speed (hereinafter, frame speed) during continuous shooting, the focus detection operation has to be finished within a predetermined time. However, the focus detection time can expand or shrink based on the shooting environment (object luminance, contrast), number of focus detection points, signal storage time at each focus detection point, reading amount of the signals, and prediction calculation amount. Consequently, the frame speed can be uneven. Especially, when continuously shooting at high speed, since the AF possible time that can maintain the frame speed shortens, the frame speed tends to become uneven.
Many technologies have been discussed that shorten the focus detection operation to maintain frame speed. For example, Japanese Patent Application Laid-Open No. 9-311269 discusses a method for shortening the time it takes for focus detection by placing an upper limit on the charge storage time in other AF sensors based on the charge storage time in the AF sensor of a selected focus detection area.
Auto gain control (hereinafter, AGC) is a technology that individually compares in the AF sensor a storage signal level with a predetermined signal level, and stops the storage operation when the storage signal has reached the predetermined signal level.
To reduce the size, lower the cost, and control the power consumption of an AF sensor, it is desirable to suppress as much as possible unevenness in the frame speed even when a storage signal cannot be immediately read after the charges are stored.
In AF sensor storage/AGC control, it is necessary to guarantee the focus detection accuracy by finishing the AGC control of the plurality of AF sensors after waiting for a predetermined time from storage of the charges in each line sensor has started until they are read, and then performing reading control of the charges.
FIG. 21A illustrates the storage/AGC control illustrated in FIG. 20 when the storage signals from line sensor A are read before storage of line sensor B is completed. Even if the AF sensor completes storage of line sensor B by stopping AGC control of line sensor B, during the reading of line sensor A, the AF sensor cannot stop storage. Therefore, the signals stored in line sensor B can be saturated in the period from the reading of line sensor A until AGC control restarts. Accordingly, as illustrated in FIG. 21B, saturation of line sensor B can be prevented by setting the AGC control waiting time longer, expecting that the storage stop processing will be performed appropriately even if storage of line sensor B is completed at the timing illustrated in FIG. 21A.
Japanese Patent Application Laid-Open No. 8-152551 discusses an AF sensor in detail.
However, because the focus detection accuracy is guaranteed, the focus detection time lengthens, so that frame speed unevenness tends to occur (FIG. 5). Further, unevenness in the frame speed also tends to occur when the continuous shooting frame speed has increased because the time that can be spent on the focus detection time shortens.
Further, if the number of focus detection points is large, it takes time to carryout the storage/reading of charges according to the number of AF sensors assigned to the focus detection points (FIG. 8). Consequently, since the focus detection time becomes longer than the current focus detection time, unevenness in the frame speed tends to occur.
In a camera that can be set by a user to focus priority or shooting speed priority, if the same focus detection control is performed for focus priority and shooting speed priority, because the same focus detection accuracy is maintained, the focus detection time lengthens, so that unevenness in the continuous shooting speed (hereinafter, frame speed) tends to occur (FIG. 11).
There is trade off between maintaining a permissible maximum AF possible time and frame speed. Consequently, it is necessary to determine based on a user's shooting setting whether to emphasize the permissible maximum AF possible time or emphasize maintaining frame speed.